Synthesis of N,O-rich active site porous polymers and their efficient recovery of Gd(Ⅲ) from solution

Our previous study found that the adsorption performance of porous carbon for Gd(Ⅲ) could be significantly improved by increasing the N,and O functional groups on its surface.Unfortunately,the adsorption capacity of porous carbon is low due to the limited number of N,and O functional groups that can be loaded on its surface.Due to the advantage of customizable functional groups of porous organic polymers(POPs),the triazine-based Tb-MEL and hydroxyl-modified triazine-based Tp-MEL were synthesized by a one-step hydrothermal method using nitrogen-rich melamine as the raw material and triformylbenzene(Tb) and 2,4,6-triformylphloroglucinol(Tp),respectively.The Gd(Ⅲ) adsorption isotherms and kinetic curves of Tb-MEL,and Tp-MEL at pH=6 and T=25℃ are in high agreement with the Langmuir isotherm(R^(2)>0.998) and pseudo-second-order kinetics(R^(2)>0.993),respectively.The fitted maximum adsorption capacity(q_(max)) is about 136.05 mg/g,much higher than 58.38 mg/g of Tb-MEL.The adsorption efficiency for Gd(Ⅲ) at pH=7 is close to 100%,and Tp-MEL maintains 62.4% of the initial adsorption capacity after ten cycle s of sorption/desorption of Gd(Ⅲ),with an average elution efficiency of more than 90%.Adsorption selection performance tests show that Tp-MEL has good adsorption selectivity for HREE ions,and the selectivity is as follows:Lu^(3+)>Y^(3+)>Gd^(3+)>Nd^(3+)>La^(3+)>Al^(3+)>Ca^(2+)>Mg^(2+).Batch adsorption,Brunauer-Emmett-Teller(BET) method,and X-ray photoelectron spectrometer(XPS)tests show that hydroxyl modifications lose the specific surface area of the polymer,and the functional groups contribute more to the adsorption performance than the specific surface area.